Editing Muzzle velocity (section)

==Projectile velocity==
For projectiles in [[unpowered flight]], its velocity is highest at leaving the muzzle and drops off steadily because of [[air resistance]].  Projectiles traveling less than the [[speed of sound]] (about {{convert|340|m/s|abbr=on}} in dry air at [[sea level]]) are ''subsonic'', while those traveling faster are ''supersonic'' and thus can travel a substantial distance and even hit a target before a nearby observer hears the "bang" of the shot.  Projectile speed through air depends on a number of factors such as [[barometric pressure]], [[humidity]], [[air temperature]] and [[wind speed]].  Some high-velocity [[small arms]] have muzzle velocities higher than the [[escape speed#List of escape speeds|escape speeds]] of some [[Solar System]] [[astronomical body|bodies]] such as [[Pluto]] and [[Ceres (dwarf planet)|Ceres]], meaning that a bullet fired from such a gun on the surface of the body would leave its gravitational field; however, no arms are known with muzzle velocities that can overcome Earth's gravity (and atmosphere) or those of the other planets or the Moon.

While traditional cartridges cannot generally achieve a [[Moon|Lunar]] escape speed (approximately {{cvt|2300|m/s|disp=sqbr}}) or higher due to modern limitations of [[action (firearms)|action]] and [[propellant]], a {{convert|1|g|gr|lk=out|abbr=off|adj=on}} projectile was accelerated to velocities exceeding {{convert|9000|m/s|abbr=on}} at [[Sandia National Laboratories]] in 1994.  The gun operated in two stages.  First, burning gunpowder was used to drive a piston to pressurize hydrogen to {{cvt|10,000|atm|GPa|lk=in}}.  The pressurized gas was then released to a secondary piston, which traveled forward into a shock-absorbing "pillow", transferring the energy from the piston to the projectile on the other side of the pillow.

This discovery might indicate that future projectile velocities exceeding {{convert|1500|m/s|abbr=on}} have to have a charging, [[gas-operated]] action that transfers the energy, rather than a system that uses primer, gunpowder, and a fraction of the released gas. A [[.22 LR]] cartridge is approximately three times the mass of the projectile in question.  This may be another indication that future arms developments will take more interest in smaller caliber rounds, especially due to modern limitations such as metal usage, cost, and cartridge design.  In a side-by-side comparison with the [[.50 BMG]] (43&nbsp;g), the {{cvt|1|g|gr|disp=flip|adj=on}} titanium round ''of any caliber'' released almost 2.8 times the energy of the .50 BMG {{nowrap|1=(1 g at 10 000 m/s = 50 000 joules),}} with only a 27% mean loss in momentum.  Energy, in most cases, is what is lethal to the target, not momentum.<ref>{{cite news|last1=Brown|first1=Malcolm|title=Fastest Gun on Earth: Goals Go Beyond Planet|work=The New York Times |date=22 March 1994 |url=https://www.nytimes.com/1994/03/22/science/fastest-gun-on-earth-goals-go-beyond-planet.html|access-date=23 March 2018|ref=1}}</ref>